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1
Immunological Effects of TBE Vaccination:
Increased Expression of Transcription factor T-bet Indicates
Activation of Th1-like Cellular Immunity
Pär Andersson
Fördjupningsarbete (Scientific project), 15 hp, 2008-03-16
Department of clinical and experimental medicine (IKE), Clinical immunology
Handledare (Advisor): Prof. Jan Ernerudh
2
Innehåll
POPULÄRVETENSKAPLIG SAMMANFATTNING...................................................................... 3
ABSTRACT ........................................................................................................................................... 4
INTRODUCTION ................................................................................................................................. 5
THE TICK-BORNE ENCEPHALITIS VIRUS ............................................................................................... 5 PATHOLOGY OF TBEV ............................................................................................................................ 5 CLINICAL SYMPTOMS OF DISEASE ........................................................................................................... 6 VECTORS AND TRANSMISSION ................................................................................................................. 6 EPIDEMIOLOGY OF TBEV ........................................................................................................................ 6 VACCINATION ......................................................................................................................................... 7 THE PRINCIPLES OF IMMUNITY AGAINST VIRAL INFECTION .................................................................... 7 INACTIVATED VACCINES ......................................................................................................................... 8 ADJUVANTS ............................................................................................................................................. 8 VACCINATION BREAKTHROUGH .............................................................................................................. 9 THE ADAPTIVE IMMUNE SYSTEM .......................................................................................................... 9 CD 4 T CELL DIFFERENTIATION – TRANSCRIPTIONS FACTORS T-BET AND GATA-3 .............................. 9 AIM ........................................................................................................................................................ 11
MATERIALS ....................................................................................................................................... 11
METHODS .......................................................................................................................................... 12
STATISTICS ............................................................................................................................................ 13
RESULTS ............................................................................................................................................. 14
T-BET ..................................................................................................................................................... 14 TBET/18S BEFORE AND AFTER VACCINATION ....................................................................................... 14 GATA3 .................................................................................................................................................. 15 GATA-3/18S BEFORE AND AFTER VACCINATION ................................................................................. 15 T-BET/ GATA3 RATIO AFTER VACCINATION .......................................................................................... 16
DISCUSSION....................................................................................................................................... 17
REFERENCES ........................................................................................................................................ 21
3
Populärvetenskaplig sammanfattning.
TBE är en virussjukdom som sprids av fästingar. I takt med att fästingarna har blivit fler och
spridit sig i landet har även fallen av TBE sjukdom ökat. Det finns ingen behandling mot TBE
när en person väl smittats. I de flesta fall självläker infektionen men kvarstående problem
efter sjukdom så som olika typer av neurologiska skador förekommer och även enstaka
dödsfall. För att förebygga TBE infektion kan man använda ett så kallat inaktiverat vaccin.
Det betyder att vaccinet består av hela viruspartiklar som med hjälp av formalin har avdödats.
Detta gör att viruset inte kan dela sig inuti celler, vilket är det normala sättet för viruset att
föröka sig.
Kroppens immunförsvar brukar traditionellt delas upp i två grenar, det cellulära försvaret och
det humorala försvaret. Förenklat kan man säga att det cellulära försvaret är nödvändigt för att
döda celler som infekterats av smittämnen som förökar sig inuti celler, t.ex. virus. Det
humorala försvaret ska å andra sidan aktiveras och ta hand om smittämnen som förökar sig
utanför celler t.ex. många typer av bakterier. Det som avgör vilken del av immunförsvaret
som aktiveras vid en infektion är en komplicerad process som ännu ej är helt utredd. Man vet
dock att de så kallade T-hjälparcellerna har en central betydelse. T-hjälparcellerna är innan de
aktiveras i en så kallad naive eller 0 fas. Beroende på vilka signalämnen som finns i
omgivningen när dessa celler upptäcker ett smittämne kan de bilda antigen T-hjälparceller av
1 eller 2 typ. Bildas typ 1 T-hjälparceller kommer immunförsvaret aktivera det cellulära
försvaret och bildas typ 2 T-hjälparceller aktiveras det humorala försvaret.
Eftersom inaktiverat TBE vaccin inte delar sig inuti celler, som levande virus gör, finns det
anledning att tro att vaccinet enbart aktiver det humorala försvaret. Eftersom det allmänt anses
att det cellulära försvaret är nödvändigt för att skydda mot vissa typer av virusinfektion är det
viktigt att klarlägga vaccinationens effekt. Till exempel skulle ett felaktigt immunförsvar
kunna förklara varför vissa personer blir sjuka i TBE trots vaccination. Vår studie undersökte
immunsvaret vid vaccination genom att mäta användningen av två olika gener hos T-
hjälparceller, T-bet och GATA-3. Våra resultat visade att vaccination ökar användningen av
T-bet genen men inte GATA-3 genen hos T-hjälparcellerna. Eftersom T-bet genen används
mest i typ 1 T-hjälparceller drar vi slutsatsen att det inaktiverade vaccinet förmodligen, trots
allt, ger ett immunförsvar av cellulär typ. Detta fynd stärker uppfattningen att vaccination ger
ett gott skydd mot TBE.
4
Abstract
Tick-borne encephalitis virus is the cause of much morbidity and sometimes a fatal infection.
A vaccine based on formaldehyde inactivated virus is currently the only available way of
preventing disease. This vaccine gives a high rate of seroconversion but there are reports of
vaccination breakthrough, even in people who have demonstrated a neutralizing antibody
response. The T cell response to inactivated TBE vaccine is largely unknown, but could be of
importance for the effect of the vaccine. This study characterizes aspects of the T cell
response by investigating the expression of two transcription factors, T-bet and GATA-3 with
RT-PCR. T-bet is expressed in CD4+ T cells of the Th1 type, while GATA-3 is expressed in
CD4+ T cells of the Th2 type. Our data show that vaccination with inactivated TBE vaccine
leads to increase in expression of the T-bet gene when cells of vaccinated subjects are
cultured with TBE virus. In contrast, the expression of GATA-3 remains unaffected by
vaccination. Thus, this study suggests that the inactivated TBE vaccine leads to a Th1-like
immune response in humans.
5
Introduction
The tick-borne encephalitis virus
The tick-borne encephalitis virus (TBEV) belongs to the family Flaviviridae, genus flavivirus.
The flavivirus genus consists of more than 70 viruses including the viruses causing Japanese
encephalitis, dengue fever, yellow fever and TBE. The flaviviruses have been classified into
clusters and species based on nucleotide sequence. The major clusters are tick-borne,
mosquito-borne and non-vector-borne viruses. 1 The tick-borne cluster comprises Omsk
hemorrgagic fever and three subtypes of TBE, the European TBEV (TBEV-Eu), Siberian
TBEV (TBEV-Sib) and Far eastern TBEV (TBEV-FE). 2
The TBE viruses are positive-stranded RNA viruses of approximately 11000 nucleotides.
They have one single open reading frame, which is translated into a single polyprotein. This
polyprotein is cleaved by viral and cellular proteases into the individual proteins. 2 The result
of this process is three structural and seven non-structural proteins. The structural proteins are
assembled in the endoplasmatic reticulum of the cell into immature virions consisting of
capsid protein (C), envelope protein (E) and pre-membrane protein (prM). The immature
virion is transported through the cellular secretory pathway and just before release the prM
protein is cleaved into functional M protein and the fully functioning virus is released. 3
Infectious TBEV enters cells by receptor mediated endocytosis. The E protein of the virus is
composed of three beta barrel shaped structures. Structure III is an immunoglobulin-like
domain and thought to be the receptor binding part of the protein. The precise receptor for
TBEV cell infection has not yet been elucidated. 4 A recent study made of TBE infected
humans in Lithonia implicated a role for the mutated CCR5δ32 allele, a chemokine receptor,
but its exact role in disease is not known. 5 After endocytosis has been completed, the acidic
environment in the endosome changes the E proteins’ dimeric conformation to a trimeric
conformation, which enables it to initiate fusion of the viral envelope with the endosomal
membrane and release of the viral RNA into the cell cytoplasm. 4 The E protein is also the
major target antigen of neutralizing antibodies when a protective immune response is induced.
The mechanisms of action of the polyclonal antibodies are not known but they probably act
by preventing virus attachment or fusion of the viral envelope to the endosome. 6
Pathology of TBEV
Upon ingestion of virus from the ticks’ saliva to the human host, the virus first invades and
multiplies in the Langerhans cells of the skin and then invades macrophages, histiocytes and
fibroblasts. The virus spreads to the regional lymph nodes, liver and spleen. During this
primary viremia the virus also initiates its invasion of CNS. 6 The mechanisms of transversion
of the blood brain barrier are not yet elucidated. The possible ways of blood brain barrier
crossing include passive diffusion, transocytosis, invasion of endothelial cells, or invasion of
olfactory epithelium. The exact mechanisms of viral damage are not known but once inside
the CNS the virus induces an inflammatory response which can be seen as perivascular
infiltration of activated T cells and macrophages. Damage is spread through-out the CNS and
affects primarily grey matter where neuronal degeneration, necrosis and neurophagia are seen.
6
The damage is most pronounced in the medulla oblongata, brainstem, cerebellum and the
spinal cord. 7
Clinical symptoms of disease
Serological studies indicates that 70 to 90 % of TBEV infections in human are sub-clinical or
asymptomatic.8 In those who develop disease there is an incubation period of 4-28 days with a
median of eight days. After this time there is an acute phase where the patient has
uncharacteristic influenza-like illness with fever, joint and back pain, headache, nausea and
vomiting. 9 A second phase of disease characteristically seen after approximately eight days,
develop in about 20-30% of patients experiencing a symptomatic acute phase of TBE-Eu
infection. 8 The symptoms seen in the second phase are meningeal signs (headache and neck
stiffness), ataxia, altered consciousness, impaired concentration and memory, dysphasia,
confusion, irritability, tremor, and paralysis of cranial nerves. 10
Many patients have residual
symptoms. In one study 80 % of TBE patients had symptoms of CNS dysfunction on follow-
up 6 weeks after disease and 40 % still had symptoms at one -year follow-up. 11
Vectors and transmission
The vectors of the three subtypes of TBEV are all hematophagus ticks. The TBEV- Eu is
primarily transmitted by the hard tick Ixodes ricinus while the two other subtypes primarily
spread through another hard tick; Ixodes persulcatus. Transmission between ticks occurs both
through feeding on viraemic hosts and through co-feeding, a process which involves an
infected tick feeding in proximity to a naïve tick and the spread of virus in migratory skin
cells of the host. 10
Once infected by the TBEV the tick will stay infectious through-out its
different stages of life and will also through vertical transmission pass the virus on to its
offspring. The tick goes through three stages of development; larvae, nymph and adult stage.
Development from one stage to another requires the tick to feed. The major targets for ticks
are rodents, especially the Apodemus species, a genus of Eurasian field mice. 6 These rodents
are recognized as the major transmission host of the TBEV, while other targets of tick feeding
such as humans and larger animals; goats, cows, deer, can become infected accidentally but
play a minor role in transmission of virus between ticks. 10
Epidemiology of TBEV
The occurrence of TBEV is determined by the distribution of the tick vectors. Thus, the
TBEV is spread only during the active season of the tick which starts when the temperature
rise above 5-7°C. In the central European region this means that the TBEV is spread from
March-April until October-November when temperature declines below this level. Because of
the longer duration of the active season of ticks in central Europe, there are two peaks of
incidence of TBE infection reflecting the spring and autumn population of ticks. In
Scandinavia only one peak in TBE infection is observed and it occurs in the summer. The
geographical distribution of Ixodes ricinus is Europe, central Asia and North Africa and the
TBEV- Eu has been found in all of these regions. Ixodes persulcatus is found from the Baltic
countries in the west to the eastern parts of Russia and in these regions it acts as the major
vector for the subtypes TBEV- FE and TBEV- Sib. The annual world wide incidence of
clinical cases of TBEV is 10000- 12000. Most cases occur in Russia with 5000 to 9000 cases
annually. The country with the highest incidence rate in the world is Latvia with 26.3
cases/100 000 population annually.12
Sweden had 163 reported cases of TBE in 2006. 13 This
number increase in 2007 to 182 cases of TBE and although the majority of cases (75%) occur
7
in proximity of the Stockholm area, there are now reported cases also in Östergötland, around
the lakes Vättern and Vänern and on the island Gotland. 14
Vaccination
Currently there is no anti-viral treatment for TBEV once infection has taken place, instead
only supportive treatment is given. There is also no way of interrupting the virus in the
environment so the virus will remain endemic in areas inhabited by its vectors. Prevention by
vaccination is therefore the main option to lessen the burden of disease. In Austria a massive
vaccination program started in 1981, resulting in a vaccination rate of 86 %. This in turn has
been followed by a steady decline in TBE incidence from 700 cases annually to 54 cases in
2001. 15
There are currently two vaccines available in Western Europe. FSME-IMMUN®
(Baxter Vaccine AG, Vienna, Austria) is prepared using formalin inactivated TBEV of the
Neudörfl strain found in Austria. Encepur (Chiron.Behring, Marburg, Germany) is prepared
from TBEV of the German strain K23. Trials with FSME-IMMUN® have shown a
seroconversion rate after three vaccinations between 96 and 100 %. 16
The administration
scheme for both vaccines is three doses given intramuscularly, the first two given with a gap
of three week to 3 month apart and the third one given nine to twelve months after the second.
Thereafter, a booster dose every fifth year is recommended. 15
The principles of immunity against viral infection
In the defence against viral infection, the adaptive immune system primarily utilizes two
different responses. Antibodies produced against viral surface antigens and antigens on
infected cells act to prevent the virus from infecting more cells. CD8+ T cell response against
internal viral antigens shown on the cell surface by MHC I act to eliminate infected cells and
hence to eradicate infection. In some viral infections, such as rabies, antibodies are enough to
control the replication and spread of infection. 17
In other viral infections such as lymphocytic
choriomeningitis virus, protection can be mediated through a CD8+ T cell response alone. 18
A combination of both is sometimes necessary for protection. Passive immunization with
antibodies for arena virus infection protects against viral challenge when T cell response is
fully functioning, but not in mice lacking CD8+ T cells. 19
These examples illustrate that both
of the two arms of adaptive immunity are important for viral immunity and that the relative
importance of the humoral and cellular response depends on the infecting virus.
Antibodies are often directed towards the extra cellular domain of the surface antigen. On the
viral protein there are often many different antigen sites that can be the target of antibodies.
During a first exposure to the virus the immune system often only produces antibodies against
some of these sites. During further exposure to viral antigen the immune response will evolve
to involve more of the antigen sites. This is one explanation why it may take repeated
immunizations to get a protective antibody response, as is the case with the TBE vaccines. 20
The CD8+ T cell response can be directed towards surface proteins as well as internal
structural, regulatory and non-structural proteins. The internal structural and regulatory
proteins are often more conserved, are produced in greater number and earlier in the virus life
cycle than the surface proteins.20
This make them important especially in infections with virus
that undergo rapid mutation of surface antigens such as HIV where T cells towards the
intracellular and non structural proteins Gag, Nef and Pol are present. 21
On successive viral
8
challenges the CD8+ T cell response will expand to generate an increased number of antigen-
specific T cells. 22
When a human is exposed to a virus the first line of defence is the mechanical barriers of the
body; the skin and mucosa, also including cells and molecules of the innate immunity, which
is followed by antibodies of the adaptive immune system. All of these mechanisms work to
prevent the virus from infecting the cells of the body. Once infection has occurred a T cell
response will be invoked. Viral infected cells can be destroyed be CD8+ T cell or by antibody
and complement lysis, or by antibody mediated NK-cell activation. The ideal of an
immunisation is to invoke an antibody response large enough to completely prevent virus
from infecting cells, i.e. neutralizing the viral agents. Antibodies are most effective when the
site of invasion is distant from the target organ, since the virus needs to reach the bloodstream
to access the target and is thus exposed to the antibodies. 20
This is the case with TBE
infection, which starts in the skin of the infected host but must pass through the lymph and
blood stream to reach the CNS where the damage is done.
Inactivated vaccines
The currently used TBE vaccines are of the inactivated type. 16
Inactivated vaccines are
viruses that have been treated with formalin or other chemicals, making them unable to
replicate. In comparison with the live attenuated vaccines they are generally less potent. This
might be due to their lack of proliferatory (replicative) potential which leads to a lack of
virus-encoded proteins in the cytosol of the cells, proteins that in a natural infection would be
presented on MHC I leading to a CD8+ T cell response. 23
The advantages of inactivated
vaccines are that they are more heat stable and that they lack the risk of reversal of virulence
which is always a possibility in the case of live-attenuated vaccines. 24
The fact that inactivated vaccines only induce some parts of the immune system has been
shown. In a study by Kreil et al., mice where first immunized with a three times course of
FSME-immune and then challenged with a lethal dose of TBEV. As expected, high titters of
antibodies against protein E were induced after the immunization, which resulted in survival
rate of 100 %. However, even though viremia was not detected on challenge with live virus, a
new kind of antibody developed towards non structural protein 1 that had not developed after
vaccination. Also, TBEV specific CD8+ T cells where detected after but not before challenge
with virus. The authors concluded that the vaccine protected against disease but not against
infection. 25
Another study performed by Aberle et al. made on mice vaccinated with inactivated vaccine
found that it induced a Th2 response measured as the amount IgG1 antibodies, which are Th2
specific in mice, compared to IgG2a, which are Th1 specific. This study also found that
inactivated TBE vaccine was unable to induce a CD8+ T cell response in mice. 26
Adjuvants
To enhance the effect of vaccination, adjuvants are often added to the vaccination antigens.
Adjuvants are defined as substances that enhance the immunogenicity of antigens. 23
In TBE
vaccines the adjuvant is aluminium hydroxide (Al(OH)3) 16
In mice aluminium hydroxide
favours a polarization towards a Th2 response. 24
Some adjuvants have been shown to induce
a Th1 response when given with inactivated vaccines but it has been difficult to find any
adjuvant that can induce a CD8+ T cell response in human. 27
9
Vaccination breakthrough
Measurement of neutralizing antibodies is used as a surrogate for protective efficacy in TBE
vaccination. 28
After an immunization schedule of three vaccinations, the seroconversion has
been found to be 99.6 %. 16
In Austria, where vaccinations have been extensive, 34 cases of
TBE were reported in the vaccinated population between 1994 and 2004. Of these, a majority
(20 of 34 cases) was over 60 years of age, indicating the waning of immune efficiency with
age. 29
In a case study two patients were described having proven neutralizing antibodies of
IgG type and still being infected by TBE. 30
A severe TBE infection was reported in a 54 year
old patient who had taken the full 3 times schedule and 2 booster doses, the last one three
years earlier. Although the patient survived, several residual symptoms were reported, such as
neuropathy with stabbing pain in all limbs and persistent neuropsychological deficits. 31
The adaptive immune system
As described above the immune response against viral infection depends on the adaptive
immune systems´ two major components; the humoral and cellular immune responses. The
determination of which response that will be predominant in a viral challenge depends on the
differentiation of the CD4+ T cell. The result of this differentiation is the creation of two
kinds of CD4+ T cells; Th1 and Th2. The Th1 cell is characterized by the production of the
cytokine interferon gamma (IFN-γ) and activation of macrophages. The Th2 cell, on the other
hand, produces IL-4 and is specialized in activating B cells resulting in antibody production. 32
CD 4 T cell differentiation – transcriptions factors T-bet and GATA-3
The first requirement of CD4+ T cell differentiation is activation of the naïve T cell. This
process begins when an antigen presenting cell (APC) such as a dendritic cell (DC)
phagocytes an antigen, gets activated and migrates to a peripheral lymphoid organ. In the
lymphoid organ, e.g. a lymph node, the activated APC encounters naïve T cells. The CD4+ T
cell binds the antigen presented on a MHC II molecule of the APC and at the same time its
CD4+ protein binds the MHC II molecule. The T cell also needs a co-stimulatory signal
which is the binding of its CD28 molecule to a CD80 or CD86 molecule which is highly
expressed on APCs who have been activated.32
The signal of the T cell receptor (TCR) starts with the release of intracellular calcium, which
activates the calcium-dependent phosphatase calcineurin. This phosphatase dephosphorylates
Nuclear Factor of Activated T cells (NFAT), which then translocates to the nucleus. NFAT
activation results in chromatin remodelling which is necessary for further differentiation.
NFAT may also be involved in the acute cytokine transcription of the activated T cell as it
binds the Ifng promoter and the Il4 promoter and transcription of both of these genes are
increased on activation resulting in a slight increase of both IFN-γ and IL-4 secretion. 33
The
Il4 locus is hypermethylated in naïve T cells but during Th2 differentiation it is passively
demethylated probably through some kind of interference with DNA methyltransferase 1
(Dnmt1). Dnmt1 is the enzyme which under normal conditions methylates the different alleles
of the cell during replication. If this process is interrupted the Il4 locus will lose more
methylations for every replication, which means that every new generation of cells will
produce more IL-4. 34
Demethylation is accompanied by increased acethylation of the Il4
locus. This probably occurs due to the fact that deacetylation is normally mediated by
acetylases incorporated in the multi-subunit complex which the methyl-CpG binding protein
10
(MBD2) assembles on methylated DNA. When the DNA is less methylated, less MBD2 is
able to bind and thus, less deacetylation occurs. 35
All of these changes that occur on activation of the T cell increase the possibility of further
change but are not enough for T cell differentiation. This process is dependent on the cytokine
environment in which the T cell is activated and this in turn is believed to be largely
dependent on the cytokines expressed by the DC that activated the T cell. There are two
distinct subtypes of dendritic cells in human, one expressing IL-12 upon activation and
inducing a Th1 response while the other does not express IL-12 and induces a Th2 response. 36
IL-12 act by binding the IL-12R receptor and this activates the second messenger signal
transducers and activators of transcription 4 protein (STAT4). The effect of STAT4 signalling
is increased survival of Th1 cells and increased transcription of the Ifng gene probably
through acethylation mechanisms independent of the activation of transcription factor T–box
expressed in T cells (T-Bet). Thus, IL-12 increases the IFN-γ expression and promotes a Th1
response. Th1 cells that have differentiated produce IFN-γ for approximately 72 hours and
then the production decreases. The production is prolonged by addition of IL-12 and IL-18
indicating their role as IFN promoters. 37
IL-12 derived from activated DCs also increase IFN-
γ production from NK cells and this effect is also enhanced by IL-18.38
Th1 differentiation is also promoted by the cytokine IFN-γ. Through its second messenger
STAT1 it activates T-bet. 39
This activation results in the transcription of Ifng as well as the
downregulation of the Il4 transcription. 40
STAT1 also induces the IL-12Rβ2, which increases
the sensibility of the T cell to IL-12 stimulation. 39
The differentiation of an activated T cell to a Th2 cell may be initiated by binding of IL-4 to
the IL-4R. The second messenger of this receptor is STAT6. This protein associates with the
Il4 promoter and might thus have a direct effect on the IL-4 production. However, STAT6
also up-regulates the transcription of GATA-3. This transcription factor then binds the 3´
enhancer of the Il4 locus and this is associated with the binding of NFAT to the Il4
promoter.41
Once activated this transcription factor can autoregulate its own transcription and
it also leads to an intrinsic suppression of IFN-γ production independent of IL-4 production.42
T-bet and GATA-3 thus determine the fate of the activated T cell. If T-bet is activated it up-
regulates IFN-γ and down-regulates IL-4. GATA-3 does the opposite; it increases the
transcription of IL-4 and down-regulates IFN-γ. The cytokines that are produced in this
manner work in an autocrine way to encourage their own production as well as in a paracrine
way to promote a Th1 or Th2 environment, respectively.
11
Aim
As here described, the TBE virus is the cause of much morbidity. Vaccination has been shown
to be effective on a population basis but in cases of vaccination breakthrough, the course of
the disease may be severe. The immune response induced by inactivated TBE vaccine in
animals has been shown to be of a different kind then the one seen in natural infection. The
TBE vaccination seems to induce a Th2 response instead of Th1 and also to lack induction of
CD8+ T cells. The aim of this study is to characterize aspects of cellular immune response to
inactivated TBE vaccine in humans. Blood samples were taken prior to and after full
vaccination. The mRNA expression of the Th1 specific transcription factor T-bet and the Th2
specific transcription factor GATA-3 were measured after in vitro stimulation of T cells with
inactivated TBE virus.
Materials
The blood samples used in this study came from seventeen people attending TBE vaccination
at the Department of Infectious Medicine, University hospital of Linköping. Vaccinations
took place from 2004 to 2007. The study group consisted of twelve women and five men in
the age 29 to 77, median age 66. Out of these 17 subjects, seven were healthy, four had
allergies, two had high blood pressure, one had pacemaker, one had asthma, one was under
investigation for gastrointestinal problems and one had Hashimoto’s disease. None of the
subjects had been diagnosed with TBE or Dengue fever. Three had previously been diagnosed
with Lyme disease i.e. infection of Borrelia burgdorferi. Prior to vaccination, serum samples
were collected from all subjects. All subjects had given a serum sample before vaccination
and this showed that 14 of subjects where negative for, two had borderline values and one was
positive for TBE in the terms of antibodies as measured by ELISA. However, none of the
subjects had neutralizing antibodies towards TBE before vaccination. Vaccinations took place
according to the recommended immunization schedule. 15
Blood samples was taken before the
first vaccination and then again after the third vaccination. Although 17 subjects took part in
the study, some samples had to be excluded. Out of the seventeen subjects, only seven
patient-samples taken before vaccination could be used; in five subjects there were no stored
cells available for RNA extraction, the remaining five samples had too low RNA
concentration after RNA extraction to be used in RT-PCR. Also, in two of the subjects, one of
the stimulations was excluded due to limited RNA content. As to samples taken after
vaccination, two blood samples had to be excluded due to too low RNA concentration after
extraction, leaving 15 useful blood samples in this group. The study was approved by the
Regional Ethical Review Board in Linköping, Sweden. All subjects gave informed consent.
12
Methods Blood samples were collected using phlebotomy. Peripheral blood mononuclear cells (PBMC)
were extracted using gradient centrifugation on Lymphoprep (Medinor AB, Lidingö, Sweden)
The cells were cultured in tissue culture medium consisting of Iscoves modification of
Dulbeccos medium (Invitrogen) supplemented with NaHCO3 3,024g/l, L-glutamin(Sigma
Aldrich, Sweden) 292mg/l, pencillin (In Vitro Sweden AB, Stockholm, Sweden) 50 IU/ml,
streptomycin (In Vitro) 50 ug/ml, 100x non-essential amino acids (Invitrogen) 10 ml/l and 5%
fetal bovine serum (Sigma Aldrich, Sweden) at a cell density of 1x106/ml. From each subject
four cultures were set up and stimulated with either Influenza antigen, formalin inactivated
TBE virus (Chirion, Germany) or PHA (Sigma Aldrich, Stockholm, Sweden). In addition, one
culture was left unstimulated to serve as negative control.
After stimulation, the cells were lysed by addition of buffer RLT (RNeasy 96 RNA extraction
kit, Qiagen, Hilden, Germany), in accordance to the manufacturers protocol. The lysate were
stored at -70°C until extraction of mRNA.
The resulting material of the process described above was used in the current study. Total
RNA was extracted from the lysed cells by use of the protocol and kit RNeasy® Mini Kit
(QIAGEN AB, Solna, Sweden). Briefly, the lysed cells where homogenized and then passed
through an RNeasy Mini spin column. This column contains a membrane that the RNA binds
to. Addition of water to the spin column released the pure RNA from the membrane. RNA
quality and quantity was then assessed by spectrophotometry (ND 1000; NanoDrop
technologies, Wilmington, USA).
The total-RNA extracted in this manner was then converted through reverse transcription to
cDNA which is necessary for use in quantitative PCR. This conversion was done using the
High-Capacity cDNA Reverse Transcription Kit (Applied Biosystems, Stockholm, Sweden)
and in accordance with manufacturers’ instructions. In short, a mix containing nucleotides,
primers and reverse transcriptase was put in a 96 well plate. The Total-RNA and water was
then added in proportion to the RNA concentration so as to receive a sample of the same
concentration as the sample with the least RNA content, in this study that RNA concentration
was 22.69 ng/µL.
The cDNA samples were then used to in quantitative reverse transcription polymerase chain
reaction (RT-PCR). This method was used to investigate the expression of three genes, 18S,
GATA-3 and T- Bet. The 18S gene is a “house keeping”/ control gene and its product is part
of the ribosome, this means that it should be expressed in equal numbers in all cells
independent of the cells differentiation. The GATA-3 and T-bet genes were used as indicators
of cell differentiation. All genes samples where run in duplicates and a standard curve were
made for analyse of gene expression.
The analyse of data was made through the use of a standard curve. This curve was made with
a cell lysate that had been diluted to a ratio of 1/1, 1/4, 1/16, 1/64, 1/256. Each of these
samples had then been given an arbitrary value starting with 4096 for the 1/1 dilute to 16 for
the 1/256 dilute. A threshold value of 0.05 was used as the point of measurement. A CV
(coefficient of variance; Standard deviation / Mean x 100) of less than 15% was used as a
limit of acceptable variation of duplicate values.
13
Statistics
Data were checked to determine if they were Gaussian distributed. The results indicated that
data was not normally distributed and thus non-parametric Mann- Whitney U test was used.
Statistical analysis was made using GraphPad Prism version 5 (GraphPad Software, San
Diego, CA).
14
Results
T-bet
The expression of the T-bet gene and the 18S gene was measured with quantitative RT- PCR
in the samples taken before and after vaccination. The ratio of T-Bet to 18S was then
measured for each sample and grouped in accordance to stimulation.
No significant difference was found before vaccination between the TBE stimulated and the
un-stimulated samples when compared by Mann Whitney U test (Fig.1). Nor were there any
significant difference between the PHA and Influenza when compared with the un-stimulated
samples.
After vaccinations (Fig.2) a significant increase of the T-Bet/18S ratio was seen when the
TBE samples were compared with un-stimulated (p = 0.0005). At this occasion there was also
a significant increase in the other two groups of samples when compared with the un-
stimulated samples (Influenza p < 0.0001 and PHA p < 0.0001). The TBE samples did not
differ significantly when compared with the Influenza or the PHA samples (Influenza vs TBE
P=0.2455 and PHA vs TBE P=0.0745).
Tbet/18S before and after vaccination
T-BetBefore vaccination
Un-s
timula
ted
TBE
Influ
enza
PHA
0
2
4
6
81015202530
ns
T-b
et
/ 1
8 S
T-BetAfter vaccinations
Un-s
timula
ted
TBE
Influ
enza
PHA
0
2
4
6
8
p=0.0005
T-b
et
/ 1
8 S
Fig 1. Graphs show the median and
interquartile range before vaccination
for the Tbet/18S expression ratio.
TBE, influenza and PHA samples did
not differ significantly from Un-
stimulated samples.
Fig 2. The T-bet/18S expression ratio
of the TBE stimulated samples was
significantly higher than the un-
stimulated samples after vaccinations.
The influenza and PHA samples also
differed significantly from the un-
stimulated samples but not from those
stimulated with TBE.
15
GATA3
The GATA-3/18S ratio was calculated in the same manner as for T-Bet/18S (Fig.3 and 4). No
significant differences were found before vaccination with any of the three stimulations when
compared with the un-stimulated samples.
The TBE stimulated samples GATA-3 / 18S ratio after vaccination did not significantly differ
from the un-stimulated samples. Neither did the influenza stimulated samples, whereas the
PHA samples differed significantly from the un-stimulated (P=0.0035).
GATA-3/18S before and after vaccination
GATA-3Before vaccination
Un-s
timula
ted
TBE
Influ
enza
PHA
0
2
4
6
8
10
GA
TA
-3 / 1
8S
Un-s
timula
ted
TBE
Influ
enza
PHA
0.0
0.5
1.0
1.5
2.0
2.5
GATA-3After vaccination
nsG
AT
A-3
/ 1
8S
Fig 3. There were no significant
difference in the GATA-3/18S ratio
between the samples stimulated with
TBE, Influenza or PHA and the un-
stimulated samples before vaccination.
Fig 4. The TBE and the influenza
stimulated samples GATA-3/18S
ratio did not differ significantly
from the un-stimulated samples after
vaccination whereas the PHA
stimulated samples did P=0.0035.
16
T-bet/ Gata3 ratio after vaccination
As shown above, there was a significant increase in the T-bet expression but no significant
difference in the GATA-3 expression when comparing the TBE stimulated and the un-
stimulated samples after vaccination. To directly compare the difference in expression of the
two transcription factors after vaccination, and thus show which transcription factor that
dominated after vaccinations, a calculation was done using the formula;
Subject X: TBE (T-bet/18S) / un-stimulated (T-bet/18).
This calculation was done for each of the 15 subjects and the median and inter-quartile range
of these results was calculated and compared with the same calculations for the GATA-3 and
18S expression.
Sample X: TBE (GATA-3/18S) / un-stimulated (GATA-3/18S).
The result of these calculations makes it possible to directly compare the expression of the
two transcription factors. The change in T-bet expression (median ratio of 1.89) was
significantly higher (p < 0.0001) than the change in GATA-3 expression (median ratio of
1.09) after vaccination (Fig.5).
GATA3 TBE/Un-stimulated & Tbet TBE/Un- stimulatedAfter Vaccination
GATA
3/18
S: T
BE/U
n-stim
ulate
d
Tbet/1
8S: T
BE/U
n-stim
ulate
d
0.0
0.5
1.0
1.5
2.0
2.5
p=<0.0001
Fig.5. This graph shows the
relative difference between
the TBE stimulated and the
un-stimulated samples for
each of the two transcription
factors. The median ratio was
1.89 for T- bet and 1.09 for
GATA-3 with a P-value of
<0.0001.
17
Discussion
The aim of this study was to characterize aspects of the cellular response to a three time
vaccination schedule with inactivated TBE vaccine in humans. The results showed an increase
in the expression of the T-bet gene when cells from vaccinated subjects were cultured with
inactivated TBE virus compared with un-stimulated cells from the same subjects. In contrast,
the expression of GATA-3 did not increase significantly when cells from the vaccinated
subjects were stimulated with TBE. Finally, the difference in expression of the two
transcription factors was compared to see which of the two that dominated, showing a
significant predominance of T-bet over GATA-3. As T-bet is a Th1 specific transcription
factor the interpretation of these results is that vaccination with inactivated TBE vaccine gives
a T cell differentiation mainly of the Th1 type.
A weakness of this study was the inability to collect enough data before vaccination. Data was
therefore unable to show a significant difference between the positive control (PHA
stimulation) and the un-stimulated samples in the test subject before vaccination. This makes
it difficult to exclude for certain whether the subjects of this study had a T cell response
towards TBE antigen before the vaccinations took place. The reason for this is that although
TBE stimulated cells did not differ from the un-stimulated cells in their expression of
transcription factors before vaccination, neither did they differ from the positive control.
Theoretically they could therefore have had a response towards the TBE antigen, which could
not be detected in the test in line with the unexpectedly low PHA response. There is of course
little reason to believe that the general population in Sweden would have a T cell response
towards TBE before vaccination, and also all but one subject was negative for antibodies
towards TBE before vaccination, indicating a lack of immune response. Still, this is a
concern, which should be taken in to consideration when interpreting the result of this study.
Even so, the results show a significant difference between the expression of T-bet/18S
between the TBE and un-stimulated groups after vaccination. Since it is unlikely that such a
difference should exist in the un-vaccinated population in Sweden, our conclusion is that TBE
vaccination most likely induces a T cell differentiation with an up-regulation of the T-bet
gene. This notion is further supported by the absence of GATA-3 expression.
The reasons for a low RNA concentration in samples taken before vaccination are obscure. It
should be possible to store cDNA at -70 °C. Still, it is possible that the longer storage time of
the samples taken before vaccination was the reason for the low RNA concentration seen in
some of those samples.
Another methodological consideration is the choice to look at transcription factors as an
indicator of T cell differentiation. One must keep in mind that secondary modulations later in
the signalling pathway may modify the final results of transcription factors and it could be
argued that protein levels (IFN-γ or IL-4) or antibodies (IgG1/IgG3 for Th1 in humans 43
or
IgG4 for Th2 44
) are better indicators of differentiation because they are the active
immunological agents.
The result of this study, that an immunization with inactivated TBE vaccine gives a Th1-like
immune response might seem unexpected, contradicting previous evidence of a Th2 response
to inactivated TBE vaccine found in experiments made in mice. Aberle et al. investigated the
nature of immune responses in mice immunized with different kinds of TBE vaccines. Of the
18
five tested vaccines, one was inactivated, two where live- attenuated in which parts of the
viral genome had been removed, while the two remaining vaccines were of a novel kind
consisting of the viral RNA genome with the exception of a sequence corresponding to 62
amino acid residues in protein C, thus producing non-infectious sub-viral particles lacking a
functional capsid protein necessary for virulence. After immunization, the antibody response,
which in mice distinctly mirrors the Th1/Th2 response, as well as the CD8+ T cell response,
was measured. All types of immunizations resulted in 100 % seroconversion, i.e. all mice had
TBEV specific antibodies. The IgG2a/IgG1 rate was used as a measure of the Th1/Th2
deviation of the response. Live attenuated and RNA vaccines both gave an IgG2a dominated,
i.e. a Th1-like response. Inactivated vaccine, on the other hand, gave an IgG1 dominated Th2-
like response. TBEV specific CD8+ T cells taken from the spleen of the immunized animals
were measured. The result showed that live- attenuated and RNA vaccines gave a CD8+ T
cell response while the inactivated vaccine did not. 26
Are these data totally contradicting the results of the present study or is there a possible
explanation for the diverging results? One explanation of this difference could be an essential
difference in the regulation of T cell differentiation in murine and human species, e.g.
regarding the role of DCs.
DCs do play an important role in T cell differentiation. Not only do DCs perform the
necessary steps for T cell activation by showing the antigen on their MHC molecules and
express co-stimulatory proteins such as CD80/86, but they also produce cytokines that are
responsible for the nature of the T cell differentiation. Studies have shown that DCs belong to
different sub-classes as determined by their origin in humans and by their expression of
certain proteins in mice. In humans, differences have been shown between the DCs generated
from myeloid CD11c+ origin and those produced from plasmacytoid CD11c– origin.
45 The
myeloid DC, also called monocyte DC, or DC1 produces, after activation by the CD40L
route, large amounts of IL-12, resulting in a Th1 response in naïve CD4+ T cells in vitro. The
plasmacytoid DC, also called lymphoid DC or DC2, does not produce IL-12 after CD40L
activation, and naïve T cells cultivated with DC2 differentiate into Th2 cells. 36
Plasmacytoid
DCs do not produce IL-12 upon stimulation but may in some circumstances produce IFN-α
when activated 46
These findings are comparable with the Cd8α+ and CD8α- types of DCs
which can be found in mice. The CD8α+is able to produce IL-12 leading to a Th1 response,
while the CD8α- is unable to produce IL-12 and therefore leads to a Th2 response. 47
Differences in which DC population that is activated upon stimulation with inactivated TBE
vaccine could explain the different T cell responses in humans and mice. If the inactivated
vaccine activated DCs of the monocyte/ DC1 origin in humans while activating CD8α- DCs
in mice then this would result in a Th1 response in humans and a Th2 response in mice.
Factors that could influence the DC preference include route of administration, dose and
interval between doses, number of doses and the adjuvant added to the vaccine.
Another factor of importance in T cell differentiation is differences in T cell signalling that
result from the cytokines expressed by DCs. In the mouse, there is strong evidence that the
CD8α+ DCs´ ability to induce Th1 differentiation requires IL-12. 45
Since experiments
indicates that IFN-α is unable to drive Th1 differentiation and that IFN-γ is merely a cofactor
to IL-12. 48
In humans, on the other hand, the IL-12 is not the sole driver of Th1
differentiation and not the sole activator of STAT4, since STAT4 is also activated by IFN-α.49
It has also been shown that IFN-α is able to induce a Th1 response in human naïve T cells in
vitro, independent of IL-12 in contrast to the situation in mice.50
19
Thus, these fundamental differences may affect they way T cells react on inactivated
vaccines. For example, it is possible that DC macro-pinocytosis or receptor mediated
phagocytosis, of the inactivated virus particles activated the plasmacytoid DCs to produce
IFN-α that mediates a Th1 response in humans, while it does not in mice.
It should be noted that there is no consensus about the difference in IFN-α signalling in mouse
and humans. There is indeed evidence of an IL-12 independent mechanism for Th1 induction
in mice infected with certain viruses such as lymphocytic choriomeningitis virus (LCMV),
vesicular stomatitis virus (VSV), and mouse hepatitis virus (MHV). 49
Also, there are reports
that IFN-α may be of importance for Th1 differentiation in mice being necessary for IFN-γ
production in the viral infection LCMV. This IFN-γ induction was found to be STAT4
mediated and IL-12 independent. 51
Differences in expression of Toll like receptors (TLRs) in humans and in mice are another
possible explanation for differences in T cell differentiation. Toll-like receptors are of
importance for activation of DCs and their expression of IL-12 to pathogens. Knock-out mice
lacking the MyD88 protein, a second messenger in Toll-like receptor signalling, inhibited the
Th1 response to Toxoplasma gondii which is normally produced in infection with this
organism. 52
The TLRs -3, -7 and -9 are all involved in IFN- α and -β production in DCs to
viral infection. 53
Lundberg et al have found that TLR-3 signalling in response to dsRNA
differ in mice and humans, inducing TNF-α and IL-6 in mice but not in humans. Differences
in expression of IFN- α was unfortunately not investigated in this study. 54
These are some possible mechanisms, which could explain the species difference found
between the result of this report and earlier investigations of T cell differentiation in mice.
Further studies are needed to investigate the nature of the T cell differentiation. Important
questions include; how does inactivated vaccine induce a Th1 response in humans? Is there a
real difference in T cell response to cytokines in mouse and human and what are the causes of
these differences? Do DCs respond differently to inactivated vaccine in mice and humans and
what mechanisms do DCs use to recognize the inactivated virus, i.e. what type of TLR or
other pattern recognition receptor is used? Does an inactivated vaccine give a CD8+ T cell
response in humans and if so how is the antigen showed on the MHC I molecule?
With the results presented here, we are hopeful regarding the use of inactivated vaccines in
immunization against infectious diseases requiring a Th1 response. Other data from this
vaccination study show that TBE vaccination induces both CD4+ and CD8+ T cells (Jarefors,
Ernerudh et al., unpublished data). Thus, data question the current paradigm that inactivated
vaccines are unable to induce a sufficient CD8+ T cell response due to lack of intracellular
replication. The findings put emphasis on the fact that T cell differentiation is not mediated by
MHC I expression but by cytokines and DCs and that these cells may recognize and initiate a
Th1 response to inactivated virus even in the absence of intracellular replication. Given the
induction of Th1 differentiation by the inactivated TBE vaccine there is no reason that it could
not also induce a CD8+ T cell response in humans as well. There is evidence that DCs may
present extra-cellular antigens on MHC I molecules through a mechanism called cross
presentation and that this triggers a CD8+ T cell response, a mechanism called cross-
priming.55
Whereas MHC I molecules present endogenous peptides on the surface of all
nucleated cells, the capacity of cross presentation seems to be mostly attributed to
professional antigen presenting cells (pAPCs) including B-cells, macrophages, perhaps
endothelial cells and DCs, the last cell type being the major player. 56
The focus of this
research has been mostly on experiments where viral antigens are produced in antigen donor
20
cells which enable the binding of heat-shock proteins and the transfection of these complexes
to DCs, a scenario quite different from the one that takes place when inactivated whole
vaccine is given as immunization. Experiments have never the less shown that different viral
proteins can give a cross presentation response without involvement of de novo synthesis.
This process is far from elucidated, but at least one endocytosis receptor, the mannose-
receptor, has been shown to give a soluble protein access to the cross presentation pathway in
vivo. 57
The TLRs have been shown to be of major importance in cross presentation and TLR
9, which recognizes bacterial DNA (CpG motifs) is able in combination with Gp120, a major
HIV surface protein, to induce CD8+ T cell response towards Gp120. 58
Also TLR 3 ligation
with dsRNA has been shown to induce cross presentation of ovalbumin (OVA) proteins in
DCs. 59
These data indicates possible mechanisms by which viral antigens can induce a CD8+
T cell response in humans with-out intracellular replication. If such a response is induced by
inactivated TBE vaccine can not be entirely proved by data from the current study, but
additional data indicates that TBE vaccination does increase a TBE-specific CD8+ T cell
response, as measured by increased proliferation after co-culture in vitro (Jarefors, Ernerudh
et al., unpublished data). If so, then the exact mechanism would be of interest for further
vaccine development. If further studies, on the other hand, shows that a CD8+ T cell response
is not induced by inactivated vaccine, then the experiments accounted for above give clues of
possible adjuvants that might trigger a cross presentation mechanism, for example addition of
dsRNA or bacterial DNA to the vaccine.
If vaccination with TBE gives a Th1 response as this study suggests and maybe also a CD8+
T cell response as future studies will have to investigate, then the reasons for vaccination
breakthroughs must be looked for elsewhere. One such reason could be strain differences in
the antibody inducing antigens of the TBE strain used for vaccine production and the many
strains present in the environment, such antigenically defective strains have been found in
Russia. 8 In addition, it must also be kept in mind that although a Th1-like T-bet response was
recorded at the group level, there are a number of individuals that did not mount such a
response. It remains to be elucidated whether this lack of response is due to genes, in
particular HLA genes, or if there indeed might be a response that was not detected by the
present method but could be detected for example at the protein level.
To conclude, the present study shows that TBE vaccination induces increased expression of
the Th1-related transcription factor T-bet. In contrast, no increased expression was found for
the Th2-related transcription factor GATA-3. These data suggest that TBE vaccination leads
to Th1-like memory response. Further studies, e.g. at the protein level, are needed for
confirmation.
21
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